Where a communication line, e.g., a telephone line extends outside a switching center, both the line and the connected equipment are potentially subject to foreign potentials through 60 Hz power line induction and accidental crosses. Additionally, telephone switching systems utilize high current and high voltage signaling sources for ringing, coin control, etc. For example, in selective ringing telephone systems 88 VRMS 20 Hz ringing voltage superimposed on negative and positive 48 volt battery are utilized for alerting subscriber stations. The potentials on such lines may reach +175 volts and -175 volts and switches in a network utilized to connect such signals to a line may be subjected to this entire difference in potential of 350 volts. Longitudinal voltages caused by induction from 60 Hz electrical transmission lines add to the requirement for high breakdown voltage in crosspoints. Taking into consideration the above effects of selective ringing and longitudinal signals, the crosspoints of a network should have a breakdown potential, of either positive or negative polarity, in excess of 425 volts or more.
The current carrying capacity requirement of a crosspoint in a telephone system is determined by: the talking line feed current (possibly as high as 100 milliamps); the current caused by coin control signals (60 to 100 mA); longitudinal currents (in the order of plus or minus 30 milliamps); and ringing current peaks up to 100 mA or more. The above factors indicate that a crosspoint for a telephone switching system should be able to handle on the order of 150 milliamps.
Metallic crosspoints because of their favorable electrical characteristics have been widely used as switching elements in communication switching systems. Metallic switches are capable of withstanding high current and high voltage, they have low impedance bilateral transmission characteristics and there is separation of the control and transmission lines in such switches.
Semiconductor devices, other than PNPN structures, generally are lacking in physical characteristics necessary to withstand the above-described conditions of circuit application. However, PNPN structures, while capable of withstanding high current and high potential, have many operating disadvantages. PNPN devices can be inadvertently turned-ON by rapid transient changes in voltage on the anode or cathode of the device making reliable operation of a PNPN network in a noisy, unconditioned environment very difficult. Also, the usefulness of PNPN crosspoints are limited by the fact that they cannot break current (except for special gate turn-OFF PNPNs which typically require large control voltages and currents to effect turn-OFF). Finally, the self-latching characteristic of PNPNs cannot be utilized in the proposed applications because PNPNs would turn-OFF with a current reversal such as occurs, for example, during ringing.